A method of this type is used, in particular, in high availability networks wherein frames are transmitted with redundancy. Fundamentally, the requirements that have been fulfilled in conventional Profibus networks must also be capable of being realized in current and future Profinet networks. This means that equipment exchange or the extension of a network can be carried out whilst the rest of the network continues operating (i.e. no loss of the provider-consumer relationships). This has the consequence that a conventional Profibus line (with bus architecture) corresponds in the Profinet environment (point-to-point architecture) to a ring topology.
Where, with a Profibus line, a subscriber must transmit a frame only once, and reaches all other Profibus subscribers therewith, in the modern Profinet environment, a cyclical frame is transmitted in addressed-based manner, in both directions on the ring, and therefore twice. At the feed-in node (switch), it is then ensured that this frame is filtered from the network. This prevents circulating telegrams in the ring (F-bit mechanism).
Present-day applications in the production and manufacturing environment therefore require a production unit (“machine ring”) which is equipped with Profinet communication, as a ring structure and therefore having high availability. A plurality of these production units should be linked to one another by an overarching “factory ring”. The linking itself must also be implemented with redundancy.
In order to meet the requirements for high availability of a network, the individual rings must be connected together into linked ring structures. This means that frames from one ring must be fed out and then fed in again. However, with the redundancy procedures used today for cyclic communication, the original frame circulating in a simple ring and its duplicate are both fed out. Therefore, in the adjoining ring, the original frame and the duplicate frames are doubled via both ports. This results in four identical cyclic frames in the subsequent ring structure. In general, where a plurality of such rings are connected one behind the other, the number of cyclic frames doubles with each ring topology, and the available bandwidth is halved with each further ring that is connected. As a consequence, given the orders of volume required, cyclic communication cannot be carried out in this way.
The frames circulating in the ring are filtered out of the ring at the feed-in node using the conventionally implemented F-bit mechanism. This means that each cyclic frame fed into the ring must be learned in a memory of the feed-in and coupling switches and, on being received anew, filtered again at the feed-in or coupling switch. Suitable resources must be provided for this, since otherwise feeding in of the frame is not possible.
A highly available network also means that the coupling nodes between the rings are implemented with redundancy. I.e. the ring structures are linked to one another at two or more sites (redundant coupling). In redundantly coupled ring structures of this type, the cyclic frames are fed multiple times into the subsequent ring structure via the two coupling paths and transmitted there via both ports of the coupling switch. The cyclic frames of one direction become extinguished at the other redundant coupling node (through the F-bit mechanism). Nevertheless, doubling of the number of frames takes place at the subsequent coupling nodes and thus, as with simply coupled rings, a halving of the bandwidth for cyclic Profinet communication takes place.